[Greg Ross]

"if (PWM_n1 = PWM_n2 = 0)" is not what you want. That says "set PWM_n2 equal to zero, and then set PWM_n1 equal to PWM_n2". (Actually, it sets PWM_n1 equal to the value of the expression "PWM_n2 = 0" which happens to be the value assigned to PWM_n2.) The value of the expression "(PWM_n1 = PWM_n2 = 0)" is zero (which is false, by definition), so the next statement will never get executed.

I guess what you want is "if (PWM_n1 == 0 && PWM_n2 == 0)"

P.S.
you might be tempted to try "if (PWM_n1 == PWM_n2 == 0)" which is incorrect also. This is going to compare PWM_n1 with the boolean expression "PWM_n2 == 0", so your conditional statement will be executed only if PWM_n2 is zero and PWM_n1 is 1 (or whatever value the C18 compiler uses for "true") or if PWM_n2 is non-zero and PWM_n1 is zero.